Hepatitis B virus (HBV) infects 400 million people worldwide and causes hepatitis, cirrhosis, and liver cancer. HBV related liver diseases are the outcome of complex interplay between the virus and its host. Core promoter mutants are prevalent HBV variants with reduced HBeAg expression, which replace the wildtype strains in the late HBe and anti-HBe stages of infection. Such mutants have been implicated in fulminant hepatitis. The replication capacity of core promoter mutants remains controversial. The current approach introduces the most common core promoter mutations at 1762/1764 into wild-type HBV genome, and is thus flawed in omitting other core promoter mutations and co-evolved mutations elsewhere in the genome. The novelty of our approach is to test whether naturally occurring core promoter mutants replicate more efficiently than wild-type strains. We have identified two core promoter mutants (4B and 3.4) that replicated at 10 and 5 fold higher levels than wild-type isolates, respectively. Both harbored 1762/1764 and additional mutation(s). 4B secreted viral particles to culture supernatant at 4 times higher efficiency than 3.4. Moreover, 4B secreted both enveloped viral particles and naked core particles while 3.4 failed to secrete enveloped particles. The present study plans to further characterize the replication and secretion phenotypes of naturally occurring core promoter mutants. First, we will study the replication capacity of naturally occurring core promoter mutants. We plan to establish whether high replication capacity is a common feature of naturally occurring core promoter mutants. The sequence responsible for enhanced replication of 4B will be narrowed down by chimeric constructs with a low replicating clone. In a separate approach, the contribution of core promoter mutations to the high replication phenotype will be verified directly. Second, the mechanism for different secretion phenotypes of 3.4 versus 4B will be determined. The responsible sequence element will be mapped by mosaic constructs between 3.4 and 4B. Whether altered secretion is mediated by variant surface or core protein will be determined. The viral replication and secretion as studied here are basic features of the virus with profound effect on disease severity, response to therapy and recovery. Increase in replication and decrease in secretion may help induce severe liver diseases including fulminant hepatitis.